The following is GOD’S promise to the tribes of Gad, Reuben, and the half tribe of Manasseh. They had chosen to stay on the east side of Jordan, but made the promise that they would cross Jordan and fight for the land alongside their brethren.
This is both a promise for their keeping their oath; as well as warning to what happens if they do not.
“And Moses said unto them, ‘If ye will do this thing, if ye will go armed before the LORD to war, and will go all of you armed over Jordan before the LORD, until He hath driven out His enemies from before Him, and the land be subdued before the LORD: then afterward ye shall return, and be guiltless before the LORD, and before Israel; and this land shall be your possession before the LORD.
But if ye will not do so, behold, ye have sinned against the LORD: and be sure your sin will find you out.
Build you cities for your little ones, and folds for your sheep; and do that which hath proceeded out of your mouth.’ And the children of Gad and the children of Reuben spake unto Moses, saying, ‘Thy servants will do as my lord commandeth. Our little ones, our wives, our flocks, and all our cattle, shall be there in the cities of Gilead: but thy servants will pass over, every man armed for war, before the LORD to battle, as my lord saith.’ So concerning them Moses commanded Eleazar the priest, and Joshua the son of Nun, and the chief fathers of the tribes of the children of Israel: and Moses said unto them, ‘If the children of Gad and the children of Reuben will pass with you over Jordan, every man armed to battle, before the LORD, and the land shall be subdued before you; then ye shall give them the land of Gilead for a possession: but if they will not pass over with you armed, they shall have possessions among you in the land of Canaan.’ And the children of Gad and the children of Reuben answered, saying, ‘As the LORD hath said unto thy servants, so will we do. We will pass over armed before the LORD into the land of Canaan, that the possession of our inheritance on this side Jordan may be ours.’ And Moses gave unto them, even to the children of Gad, and to the children of Reuben, and unto half the tribe of Manasseh the son of Joseph, the kingdom of Sihon king of the Amorites, and the kingdom of Og king of Bashan, the land, with the cities thereof in the coasts, even the cities of the country round about.” Numbers 32:20-33 (KJB)
Why they wanted to stay on the east side rather than cross into the Promised Land and live; I have no idea. Sure it was vast and good for cattle which they had much of. God honored their request though. They kept their promise too (Joshua 22:7-9).
God the LORD honors His word every time.